The present invention pertains to the field of devices which provide electrical connection between coaxial transmission lines and microstrip transmission lines.
Ideally, such a device, or "launcher," allows high-frequency signals to pass therethrough without causing reflections and corresponding power loss. Signal reflections are minimized by matching the characteristic impedances and electro-magnetic field configurations of both the coaxial transmission line and the microstrip transmission line.
Some early launchers are described in Lehrfeld, U.S. Pat. No. 3,553,607, Napoli, et al., U.S. Pat. No. 3,686,624, and Brown, U.S. Pat. No. 3,725,829. In each of these early launchers, an extension of a coaxial transmission line's center conductor is forced against a microstrip transmission line's upper conductor, and an extension of the coaxial transmission line's outer conductor electrically couples with the microstrip transmission line's lower conductor.
In each of these launchers, the electromagnetic field configuration changes abruptly at the point of contact with the microstrip transmission line. Bogar, U.S. Pat. No. 3,705,379, shows a launcher which lessens the abrupt change in electro-magnetic field configuration. An extension of the center conductor of the coaxial transmission line has a lanced-out portion which is offset downward relative to the extension of the outer conductor. The launcher of Davo, U.S. Pat. No. 3,622,915 uses a ramp-like member to further reduce any discontinuities in electromagnetic field configuration. However, both the Bogar and the Davo devices improve the electromagnetic field distribution at the expense of losing the constant characteristic impedance.
A launcher is disclosed in Eisenhart, U.S. Pat. No. 4,280,112, the purpose of which is to eliminate discontinuities in both the characteristic impedance and the electromagnetic field configuration. By gradually displacing a center conductor contact pin of gradually diminishing diameter from the center axis of an outer conductor of constant diameter, the launcher maintains a constant characteristic impedance while transforming the electromagnetic field configuration between a coaxial transmission line configuration and a microstrip transmission line configuration. Such device, however, does not solve all of the problems facing launcher design. The launcher is optimized for a microstrip transmission line having a specific dielectric substrate thickness; if used with a microstrip transmission line having a different dielectric substrate thickness, capacitive discontinuities occur at the junction, resulting in greatly decreased performance. Moreover, there is no way to compensate for manufacturing variations of pin and bore diameters. The launcher also has a cantilevered contact pin tip with a flat contact surface to provide maximum surface area to contact the upper conductor of a microstrip transmission line. Due to manufacturing variations, the actual contact surface may be either an edge (caused by slight axial twisting of the contact pin) or three points of the contact pin's lower surface. These contacts yield different results, yet are not easily repeatable. Finally, mass production of the cantilevered contact pin is difficult because the end of the pin opposite the contact tip must be drilled and tapped at a precise angle relative to the pin's axis.